The invention relates to a method of controlling a fuel system for use in delivering fuel to an internal combustion engine. In particular, the invention relates to a method of controlling the fuel system so as to provide a limp-home capability in the event that a fault condition occurs within the system. The invention also relates to a fuel system arranged to provide a limp-home capability in the event of such a fault.
A common rail system typically includes a source of fuel in the form of a common rail which is charged with fuel at high pressure by means of a high pressure fuel pump. The common rail delivers fuel to a plurality of injectors, each one being arranged to inject fuel into an associated engine cylinder. The common rail is provided with a rail pressure sensor providing an output signal indicative of the pressure of fuel within the common rail and, hence, the pressure of fuel delivered to the injectors.
The quantity of fuel to be injected during an injection event is calculated by means of an appropriately programmed control unit in response to a driver demand signal and other operating conditions of the engine, for example speed and temperature. The quantity of fuel delivered during an injection event depends upon both the pressure of fuel within the common rail and the duration for which an injection occurs.
It is known to provide the fuel system with a pressure regulating valve arranged to control the pressure of fuel supplied to the high pressure fuel pump and, hence, the pressure of fuel within the common rail. The pressure of fuel within the common rail can be varied by varying the current supplied to the pressure regulating valve in response to the output signal from the rail pressure sensor.
The supply of current to the pressure regulating valve is varied by the control unit in response to the pressure sensor output signal so as to ensure the required rail pressure is maintained.
If a fault occurs in the rail pressure sensor, this method can no longer be used to maintain operation of the engine. However, by controlling the current supplied to the pressure regulating valve, it is possible to provide a vehicle limp-home capability as the current supplied to the pressure regulating valve is related directly to the pressure of fuel within the common rail. Therefore, if the pressure sensor fails, the engine can still be operated sufficiently to enable the vehicle to be driven to a service center for repair.
If the fuel system is not provided with a pressure regulating valve, it is not possible to control operation of the engine in this way in the event that a fault occurs in the rail pressure sensor. In such systems, failure of the rail pressure sensor causes engine operation to be halted, leaving the vehicle immobilized until the fault can be corrected.
It is an object of the present invention to provide a method of controlling a fuel system such that engine operation can be maintained if a fault occurs in the pressure sensor, even if the fuel system is not provided with a pressure regulating valve.
By way of background to the present invention, U.S. Pat. No. 5,937,826 describes a control system for an internal combustion engine in which a low pressure pump supplies a pressure regulated supply of fuel to a high pressure pump. High pressure fuel from the pump is delivered to an accumulator under the control of ON/OFF valves, each of which controls the fuel flow from a respective pumping cylinder of the pump. The high pressure pump is controlled in response to a requested fuelling signal, an engine speed signal and a pressure output from a pressure sensor. Under normal operating conditions, the system operates closed loop such that fuel pressure within the accumulator is controlled by switching the high pressure pump valves between ON and OFF (open and closed) states in response to the requested fuelling signal, the engine speed signal and the pressure output. In the event of a fault occurring in the pressure sensor, the system operates open loop in response to a predicted pump command signal based on fuel command and engine speed. An appropriate pump command value is determined, either directly or by interpolation from a look up table, for various fuel command values and engine speed values.
According to a first aspect of the present invention, a method of controlling the operation of an engine fuel system including at least one fuel injector, a source of fuel at high pressure for delivering fuel to the injector and a pressure sensor providing an output signal indicative of the pressure of fuel delivered to the injector, comprises the steps of:
controlling the rate of flow of fuel delivered from the source to the injector by means of a metering valve arrangement supplied with a control current;
measuring the speed of the engine;
monitoring the status of the pressure sensor so as to determine whether a fault condition has occurred; and
in the event that a fault condition has occurred, varying the control current supplied to the metering valve arrangement in response to the measured engine speed so as to maintain engine operation at a substantially constant speed.
The invention provides the advantage that, even in the event of failure of the pressure sensor, operation of the engine can be maintained to provide a limp-home capability. This enables the driver of the vehicle to move the vehicle to a safe location or to a service centre. In the failure mode, the system operates closed loop by feeding the measured engine speed to determine a control current for the metering valve arrangement.
The method is particularly suitable for use in controlling the operation of a common rail fuel system comprising a common rail charged with fuel by means a high pressure fuel pump, wherein the common rail is arranged to deliver fuel to the injector.
In a preferred embodiment, the method includes the step of moving a valve member of the metering valve arrangement through a range of operating positions so as to vary the rate of flow of fuel to the high pressure pump and, hence, the pressure within the common rail.
The metering valve member is conveniently arranged to vary the extent to which an orifice in a flow path between a transfer pump and the high pressure pump is opened.
Preferably, the method comprises the further steps of providing a pressure control means for supplying the current to the metering valve arrangement, and providing an injection control means for supplying an injection current to the injector so as to control the duration for which an injection of fuel occurs.
Conveniently, the pressure control means and the injection control means form part of a control unit programmed with an appropriate control algorithm.
The method may include the step of generating a predetermined injection current to be supplied to the injector arrangement in the event that a fault condition occurs so as to set a duration for which an injection of fuel occurs, such that the quantity of fuel delivered by the injector depends only upon the control current supplied to the metering valve arrangement.
In use, if a fault occurs in the pressure sensor, the injection control means provide a predetermined injection current to the injector to control the duration for which an injection of fuel occurs. The speed of the engine is measured and, if the measured speed is less than a predetermined, demanded speed, the control current supplied to the metering valve arrangement is increased so as to increase the rate of flow of fuel from the high pressure pump to the injector. As a result, the pressure of fuel supplied to the injector is increased, thereby causing an increase in the quantity of fuel injected by the injector and, hence, an increase in engine speed. If the measured speed increases to a value greater than the demanded speed, the control current supplied to the metering valve arrangement is reduced so as to reduce the rate of flow of fuel to the injector, thereby reducing the pressure of fuel supplied to the injector. The quantity of fuel delivered by the injector is therefore reduced and, hence, the engine speed is reduced. In this way, the engine speed can be maintained at a substantially constant speed, sufficient to enable the vehicle to be driven to an appropriate location for service or repair.
Preferably, the predetermined injection current sets a predetermined duration for which an injection of fuel occurs, and is derived from a demanded engine speed which is typically greater than the idling speed of the engine.
It will be appreciated that the metering valve arrangement may be arranged such that an increase in the control current supplied to the metering valve arrangement causes a decrease in the rate of flow of fuel supplied to the high pressure fuel pump, and hence a decrease in the pressure of fuel supplied to the injectors.
According to a second aspect of the invention, a fuel system for an engine includes at least one fuel injector, a source of fuel at high pressure for delivering fuel to the injector, a pressure sensor arranged to provide an output signal indicative of the pressure of fuel delivered to the injector, a metering valve arrangement including a valve member which is movable through a range of operating positions to vary the rate of flow of fuel to the source and, hence, the pressure of fuel to be delivered to the injector, control means for controlling a current supplied to the metering valve arrangement, means for measuring the speed of the engine, a monitor for monitoring the status of the pressure sensor so as to determine whether a fault condition has occurred within the sensor, and wherein the control means is arranged to vary the control current supplied to the metering valve arrangement in response to the measured engine speed so as to maintain engine operation at a substantially constant engine speed in the event that a fault condition is detected by the monitor.
The fuel system may take the form of a common rail fuel system comprising a common rail charged with fuel by means of a high pressure fuel pump which is supplied with fuel by a transfer pump through the metering valve arrangement.
The system may include a pressure control means for supplying the current to the metering valve arrangement and an injection control means for supplying an injection current to the injector so as to control the duration for which an injection of fuel occurs.
Conveniently, the pressure control means and the injection control means form part of a control unit programmed with an appropriate control algorithm.
Other preferred and/or alternative features of the method of the present invention are equally applicable to the apparatus of the second aspect of the invention.